This invention pertains to telephone switching systems in general and to a digital trunk circuit for use in interfacing digital transmission lines and digital electronic telephone switching systems, in particular.
Where a digital transmission line is used to interconnect a digital switching network of a Central Office with remote equipment such as a remote line switch, a channel bank or another digital office, a carrier terminal unit (CTU) or span interface is used to provide the terminating functions for the transmission line. Presently, the primary digital carrier DS1 employed in North America consists of 24 pcm channels, 1.544 Mb/s bit rate and D2/D3 signaling format.
The receive section of a carrier terminal unit must perform the following functions:
(a) conversion of bipolar DS1 signals from the T1 line to unipolar return-to-zero and nonreturn-to-zero DS1 signals;
(b) extract a bit clock from the return-to-zero data stream in the presence of consecutive zeroes and pattern-induced jitter;
(c) provide bit and frame alignment;
(d) provide frame detection including in-frame monitoring, framing search and frame alignment.
It has been found particularly advantageous in one recently developed family of time-division multiplexed switching systems under development by ITT North Electric Company and called DSS to switch voice paths at a 2.048 Mb/s rate. Various aspects of the DSS family of systems have been described by N. J. Skaperda in "Generic Digital Switching System", International Switching Symposium, Vol. 1, October, 1976 by C. G. Svala in "DSS-1, A Digital Local Switching System with Remote Line Switches", proceedings of the National Telecommunications Conference, p 3915-1 1977; and by F. Fellinger in "Modular Digital Switching Network", International Communications Conference, June 4-7, 1978.
A carrier terminal unit for a system such as DSS must, in addition to providing the functions listed above, provide for converting the 1.544 Mb/s incoming DS1 data stream to the 2.048 Mb/s data rate employed in DSS as well as convert the format to that employed in DSS.